The present invention relates to an optical head used for writable-type optical disks, and to a tracking method which employs the optical head. More particularly, the invention relates to improvements in a conventional so-called tri-beam method.
Optical disks from which data can be read owing to a change in reflection factor and on which data can be written are referred to as WORM (WRITE-ONCE READ-ONLY MEMORY)- or DRAW-type optical disks. Generally, in each of these optical disks, a recording material such as Te (tellurium) is vaporized by irradiation with a laser beam, thereby to form pits in the surface of the disk. Data is read from the disk on the basis of the difference between the reflection factors (the reflection intensities) of the pitted parts and unpitted parts of the disk surface. In particular, a phase change-type optical disk has recently been developed as an erasable optical disk. In this optical disk, the phase state of a recording material is changed by the difference between the power levels of an irradiating laser beam. On the basis of the changes in phase state, changes in the reflection factors of the unchanged and changed parts of the recording material are read as data. Since the changes in the phase state are reversible, the optical disk of the phase change-type possesses an erasability property.
Meanwhile, methods of tracking ordinary optical disks, in which playback-only optical disks (also including so-called CD's) are also included, are classified into
(1) the so-called tri-beam method and PA0 (2) the so-called push-pull method.
The triple-beam method (1) is chiefly adopted for the tracking of a playback-only optical disk as stated in, for example, Japanese Patent Publication (KOKOKU) No. 58-56164. The principle of the tri-beam method is illustrated in FIG. 9A. According to this method, secondary light (corresponding to light of orders .+-.1 in FIG. 9A) is produced from a laser beam by the use of, for example, a diffraction grating, the reflected light of the secondary light from the surface of the optical disk is received by photodetectors 200, 202 shown in FIG. 9B, and a tracking servo signal is generated on the basis of the difference between the output signals of the detectors and is utilized for tracking. The servo tracking is performed so as to equalize the received quantities of light of order -1 and order +1.
On the other hand, the push-pull method (hereinafter referred to as the "PP method") (2) performs servo tracking in such a manner that, as stated in U.S. Pat. No. 3,913,076 (Japanese Patent Publication No. 55-26529), etc., a tracking servo signal is formed from an interference wave between reflected light from a pre-groove formed in the surface of the optical disk beforehand and reflected light from outside the pre-groove, whereupon an objective for focusing a laser beam on the disk is shifted in accordance with the signal. This PP method is chiefly adopted for the servo tracking of the writable optical disk.
The reason why the tri-beam method is adopted for the playback-only optical disk and the PP method for the writable optical disk in this manner is as follows: Although the triple-beam method poses no problem when playing back an optical disk already written on, a problem does arise as to tracking during the formation of pits in a data writing operation. The following is the reason:
When data is written on the recording surface of the optical disk, pits are formed anew along a track (a pre-groove) on a blank disk. Accordingly, with a boundary at a position which is being irradiated with the laser beam, the pits are already formed in the part of the track behind the irradiated position, whereas no pit is yet formed in the part of the disk ahead of the irradiated position. In consequence, when the disk has the data written thereon while being tracked by the tri-beam method, the reflection intensity I.sub.-1 of the laser beam from the part not yet pitted and the reflection intensity I.sub.+1 thereof from the part already pitted as indicated in FIG. 10B give rise to a difference (I.sub.-1 &gt;I.sub.+1 in the example of FIG. 10B) in the course of the writing operation. Therefore, even in a case where the pre-groove lies exactly midway between the two beams, an unfavorable deviation develops between the output signals (signals E and F) of the respective detectors 200, 202.
If an effort is made to use the tri-beam method also for tracking in the writing operation, the above deviation can be cancelled by applying a gain ratio (an offset value .alpha.) between the outputs (signals E and F) of the detectors 200 and 202 and then setting the tracking signal to EQU E-.alpha.F
However, since the light reflection factor of the pitted data part differs depending upon the type of optical disk, the method of applying the offset cannot produce stable tracking signals unless the deviations are compensated for by changing the offset values as, for example, .alpha..sub.1, .alpha..sub.2, . . . for the respective different materials of the optical disks. In actual practice, however, it is impossible to change the offset value for every material in this manner. That is, the application of the tri-beam method to the writing operation of the optical disk involves a first problem in which compensation cannot be made for fluctuations in the reflection factors corresponding to the materials of the optical disks.
On the other hand, if the tri-beam method is applied to playback of the optical disk already written on, the reflection intensities of the light of the orders +1 and -1 from areas before and behind a data reading pit are equal (I.sub.-1 .apprxeq.I.sub.+1) in the course of reading the pit, as illustrated in FIG. 10A. It is therefore unnecessary to provide the photodetection signals with the offset as in the writing operation. In other words, the tri-beam method is disadvantageous in that it is necessary to contrive circuitry which distinguishes between the writing mode and the reading mode and then applies the aforementioned offset only in the former. This is the second problem encountered in the tri-beam method.
On the other hand, since the PP method utilizes the interference between the reflected light from the pre-groove and from outside this pre-groove as stated before, it does not cause the offset ascribable to the difference in the reflected light intensities in the writing operation as in the case of the triple-beam method. However, since the objective is displaced for tracking as explained above, the reflected light beams from the disk shift transversely and it is eventually required to provide the servo signal with an offset magnitude. The control of this offset is complicated and is unsuited to inexpensive playback-only optical disks such as CD's. In the last analysis, the PP method is mainly adopted for optical disks requiring high precision.
Thus, both the tri-beam method and the PP method, which are the typical tracking methods, involve some disadvantages. The former is unsuitable especially for the writable (WORM type) optical disk, while the latter is suitable for the writable optical disk but leaves the problem of the lens shift unsolved.